1. Field of the Invention
This invention relates to an apparatus and process for abatement of fluorine in gases containing same, as for example in effluent gas streams produced in semiconductor manufacturing operations.
2. Description of the Related Art
The trend of recent years in the semiconductor industry has been to optimize reactors that use perfluorinated compounds (PFCs), as an approach to minimizing the presence of PFCs in the effluents of such reactor systems.
Despite the pervasiveness of this approach, there is also emerging a renewed effort to resolve the problem of PFC emissions by treatment of effluent gas streams from such reactor systems, to remove high concentrations of fluorine gas and other fluorinated organic gases from the effluent gas streams that are discharged from the reactor or other process tool.
In December 1997, over 160 countries of the world negotiated the Kyoto Climate Protection Protocol. This global agreement is intended to encourage immediate efforts to reduce the emission of greenhouse gases. Perfluorinated gases and sulfur hexafluoride (SF6) were listed among the six gases specifically targeted under the protocol. These fluorine (F) saturated species are among the strongest greenhouse gases, with global warming potentials (GWPs) 3 and 4 orders of magnitude higher than CO2. Moreover, they are extremely stable molecules with lifetimes in the atmosphere of thousands of years.
The electronics industry uses PFCs in a number of plasma processes to generate highly reactive F2 and fluorine radicals. These in situ generated species are produced to remove residue from tools or to etch thin films. The most commonly used PFCs include CF4, C2F6, SF6, C3F8, and NF3. Chamber cleans after chemical vapor deposition (CVD) processes account for 60-95 % of PFC use (Langan, J., Maroulis, J., and Ridgeway, R. Solid State Technology July, 115 (1996)).
Ongoing research to reduce PFC emission levels falls into four categories: optimization, alternative chemicals, recovery/recycle, and abatement. Process optimization was recognized by industry leaders as the preferred choice to reduce PFC emissions; abatement fell last on that list.
Process optimization involves adjusting the operating conditions in the reactor to achieve enhanced PFC conversion within the tool. Existing non-optimized conditions result in PFC utilization that vary depending on the specific gas and process used. For instance, oxide etch using a combination of CF4 and CHF3 ranks lowest with 15% efficiency. Tungsten deposition processes are reported to utilize up to 68% of NF3. Recent developments in optimized plasma clean technologies were proven to provide up to 99% NF3 utilization within the tool (Proceedings of the Global Semiconductor Industry Conference on Perfluorocarbon Emissions Control, Monterey, Calif. Apr. 7 and 8, 1998).
High PFC conversions result inevitably in the formation of hazardous air pollutants (HAPs). Breakdown products include mostly fluorine (F2) and silicon tetrafluoride (SiF4) gases and to a lesser extent HF and COF2. Destruction of fully fluorinated gases generates considerably augmented HAP yields compared to the initial PFC volumes delivered to the tool. For instance, assuming stoichiometric conversion of PFCs into F2, a 1 liter per minute (lpm) flow rate of NF3 could potentially produce 1.5 lpm of F2. The combined exhaust stream of four chambers could potentially generate tip to 6 lpm of fluorine gas resulting in a post-pump effluent concentration of 3% F2 (50 lpm ballast N2 per pump). These estimated values double with hexafluorinated PFCs (compared to NF3) and are likely to increase in the future with the projected throughputs of 300 mm wafer manufacturing.
The toxic and corrosive nature of fluorinated HAPs pose considerable health and environmental hazards in addition to jeopardizing the integrity of exhaust systems. In particular, the oxidizing power of F2 is unmatched by any other compound and is far more reactive than other halogens. The large volumes F2 and other fluorinated hazardous inorganic gases released during optimized plasma processing require the use of (POU) abatement devices in order to minimize potential dangers and to prolong tool operation. Out of all fluorinated inorganic gases, fluorine gas, F2, poses the higher challenge for its abatement and the ensuing description addresses existing alternatives for its abatement.
Current fluorine abatement alternatives include dilution, dry, thermal and wet techniques.
In dilution treatment, non-reactive gases are added to lower the concentration of fluorine and other hazardous materials in the effluent stream being treated.
At high concentrations, fluorine reacts exothermically with all elements except O2, N2, and noble gases. Consequently, a reasonable approach to F2 abatement is to remove this highly active gas using naturally occurring reactions without adding energy to the system.
In the dry abatement methods for F2 removal, the fluorine gas stream is flowed through a dry bed filled with a reactive material. Alumina has been used in the past for this purpose (J. T. Holmes et al I&EC Process Design and Development. Vol 6. No. 411 (1967) In this approach, suitable dry chemicals convert F2 into innocuous solids or benign gases without generating excessive heat, an important condition since heat generation can be a limiting factor especially if the dry chemical bed is exposed to large volumes of F2.
Thermal abatement approaches combine reactive materials and F2 inside a reactor that is heated using fuel or electrical energy. Existing thermal units require the addition of hydrogen source/fuels such as methane or hydrogen to drive the fluorine reaction to completion, converting fluorine into HF. Users do not desire adding such gases since they thereby increase hazard risk and cost of ownership of the abatement system. Further, the by-products generated by the thermal abatement of F2 typically include hot acids that in turn require the use of a post-treatment water scrubber. The removal efficiencies in these scrubbers are often compromised due to the fact that the scrubbing efficiency of most acid gases decreases as a function of increasing temperature. In addition, containment of hot concentrated acids requires expensive materials of construction to prevent temperature-enhanced corrosive attack on lines, vessels and fittings.
In wet abatement methods, the fluorine is reacted with H2O. The main products of the reaction between water and F2 are HF, O2, and H2O2. (Cady, G. H. J. J. Am. Chem. Soc. 57, 246 (1935). Objections to using water scrubbers include concerns over the formation of unwanted OF2, and the large water consumption necessary to achieve acceptable removal efficiencies at high fluorine challenges.
It therefore is apparent that all of the conventionally employed approaches to abating fluorine in effluent gas streams have associated deficiencies, which limit their commercial viability and amenability to economic and practical use.
It correspondingly is an object of the present invention to provide an improved apparatus and method for the removal of fluorine and fluorine-containing gaseous compounds from gases containing same.
It is another object of the invention to provide an improved apparatus and method of such type that is adaptable to implementation for the treatment of effluent gas streams from semiconductor manufacturing operations.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.